A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer (conductor or strand shield), an insulating layer, a second semiconducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. Additional layers within this construction such as moisture impervious materials are often incorporated. Other cable constructions such as plenum and riser cable omit the shield.
It is well known in the cable art that it is desirable to have insulation, which is crosslinkable, and superior with respect to the following properties: blooming prevention, anti-scorch, process stability, water tree retardancy, thermal distortion resistance, and heat aging resistance.
Crosslinked cables are typically prepared by extruding around an electrical conductor a compounded insulation composition comprising a polyethylene, one or more antioxidants, and a crosslinking agent, and curing same. When the insulation is processed into a cable, the resulting properties, mentioned above, are affected significantly by the kind and amount of the antioxidants, which are compounded into the insulation. Conventionally, thiobisphenol has been most widely used as the antioxidant of choice for cable insulation; however, when used as the only antioxidant, it has been known to cause various problems such as unsatisfactory heat aging resistance. This leads to blooming, which results in the deterioration of water tree retardancy because of the micro-voids produced by the blooming. On the other hand, if, in order to avoid the blooming, the compounding amount of the antioxidant is lowered, problems such as anti-scorch and process stability arise.
In order to solve this problem, it has been proposed to compound polyethylene with 4,4'-thiobis-(3-methyl-6-t-butylphenol), which is one of the thiobisphenols, and one other antioxidant as described in Japanese Patent Publications Gazette No. 9173/1987 or 36061/1987, but these combinations do not satisfactorily achieve the following properties: blooming prevention, anti-scorch, process stability, water tree retardancy, thermal distortion resistance, and heat aging resistance.